During busier days, I've heard/seen them start the signup pre-RD. But on normal days it's at Sound Dangerous. Follow the signs, in case they have moved it.
You really need to head there at RD and get signed-up, if you want to send a runner ahead to TSM you need to have the padawans with the adult doing the sign up. Because the CMs need to ask them a couple of questions.
Basically (I don't remember exact wording.)
o How old are you?
o Do you want to do this?
o Will you follow the Jedi Master instructions?